Slow decreases of ionic conductance across neuronal cell membranes, which generate slow postsynaptic potentials, can increase the effectiveness of synaptic transmission. Slow conductance decreases of sufficient magnitude increase the amplitude of monosynaptic fast EPSPs in B cells of the bullfrog sympathetic ganglia. By this postsynaptic echanism, activation of one synaptic pathway can cause an increase in transmission, lasting several minutes, across another synapse. This may provide an important mechanism for synaptic integration and control of neuronal interaction. BIBLIOGRAPHIC REFERENCES: Schulman, J.A. and Weight, F.F.: Synaptic transmission: long-lasting potentiation by a postsynaptic mechanism. Science, 194: 1437-1439, 1976. Weight, F.F. and Schulman, J.A.: Long duration potentiation of synaptic transmission by synaptically and iontophoretically activated decreases in postsynaptic membrane conductance. In Symposium on "Iontophoresis and Transmitter Mechanisms in the Mammalian Central Nervous System", in press, 1977.